Nucleic Acids and Chromosome Structure (Chapters 1–6) 9/4/25 genetics

Vocabulary-style flashcards covering the core concepts of nucleotides, base pairing, DNA structure, chromatin organization, and chromosome features discussed in Chapters 1–6.

Nucleotide

The building block of nucleic acids composed of a five‑carbon sugar, a nitrogenous base, and a phosphate group.

Nitrogenous base (DNA vs RNA)

DNA uses adenine (A), guanine (G), cytosine (C), and thymine (T); RNA uses A, G, C, and uracil (U) in place of thymine.

Adenine

A purine base that pairs with thymine in DNA (or uracil in RNA) via hydrogen bonds.

Guanine

A purine base that pairs with cytosine via three hydrogen bonds.

Cytosine

A pyrimidine base that pairs with guanine.

Thymine

DNA pyrimidine base that pairs with adenine; replaced by uracil in RNA.

Uracil

RNA pyrimidine base that pairs with adenine; replaces thymine in RNA.

Deoxyribose

Five‑carbon sugar in DNA; attaches to a nitrogenous base at the 1′ carbon and to phosphate groups to form the backbone.

Phosphate group

Phosphate moiety that links nucleotide sugars to form the DNA/RNA backbone via phosphodiester bonds.

dAMP/dGMP/dTMP

Deoxyadenosine/deoxyguanosine/deoxythymidine monophosphates; nucleotides with one phosphate group in DNA.

5′ end

End of a nucleic acid where the 5′ carbon of the sugar bears a phosphate; direction of synthesis is 5′→3′.

3′ end

End of a nucleic acid where the 3′ carbon of the sugar bears a hydroxyl group; growth occurs toward this end.

Nucleotide linkage (phosphodiester bond)

Bond between the 3′ hydroxyl of one sugar and the 5′ phosphate of the next, creating the backbone.

Antiparallel

Two DNA strands run in opposite 5′→3′ directions in the double helix.

Double helix

Two antiparallel DNA strands with paired bases inside and a sugar–phosphate backbone outside; proposed by Watson and Crick.

A–T and G–C base pairing

A pairs with T via two hydrogen bonds; G pairs with C via three hydrogen bonds in DNA.

Hydrogen bond

A weak, noncovalent interaction stabilizing base pairs in DNA; 2 bonds for A–T, 3 bonds for G–C.

GC content

Proportion of G and C bases in DNA; higher GC content raises DNA melting temperature due to more H‑bonds.

Rosalind Franklin

Scientist whose X‑ray diffraction data helped reveal DNA’s dimensions and helical structure.

Watson–Crick model

Proposed the DNA double‑helix structure based on Franklin’s data and other evidence.

Width of DNA (2 nm)

Measured diameter of the DNA double helix; consistent with purine–pyrimidine pairings.

Pyrimidine

Single‑ring bases (C, T, U).

Purine

Double‑ring bases (A, G).

Primary, Secondary, Tertiary structure

Primary: nucleotide sequence; Secondary: double helix or other base-pairing patterns; Tertiary: higher‑level packing in the cell.

SNP (Single Nucleotide Polymorphism)

A single‑nucleotide difference between individuals or species in a DNA sequence.

Indel

Insertion or deletion of nucleotides relative to another sequence.

A‑form and Z‑form DNA

A‑form: right‑handed, wider; Z‑form: left‑handed, 12 bp/turn; occur under certain conditions (e.g., dehydration, GC richness).

Beads‑on‑a‑string

Description of DNA wrapped around histones to form nucleosomes, the basic unit of chromatin.

Nucleosome

DNA wrapped around a histone octamer core (H2A, H2B, H3, H4) with H1 acting as a clamp.

Histone proteins

Proteins around which DNA is wrapped to form chromatin; core histones H2A, H2B, H3, H4 and linker H1.

Chromatin

Complex of DNA and histone proteins; packaging form of DNA in the nucleus.

Euchromatin vs. Heterochromatin

Euchromatin: less condensed, gene-rich and transcriptionally active; Heterochromatin: densely packed, gene-poor and less active.

Telomere

Chromosome ends composed of repetitive DNA sequences that stabilize ends and protect against degradation.

Centromere

Chromosome region where sister chromatids attach; often rich in satellite DNA.

Satellite DNA (SAT DNA)

Short tandem repeats near centromeres and telomeres; associated with heterochromatin.

Karyotype

Visual arrangement of chromosomes used to study chromosome number and structure.

Hairpin (single‑stranded RNA/DNA structure)

A secondary structure where a single strand folds back on itself to form a loop and a paired stem.

Topoisomerase

Enzyme that modulates DNA supercoiling by cutting and rejoining DNA strands.

Supercoiling (positive/negative)

Overwinding (+) or underwinding (−) of the DNA helix; affects compaction and replication.

Denaturation

Separation of the two DNA strands, typically by heat; GC‑rich sequences denature at higher temperatures.